USB charging is very common in portable electronics today and USB ‘power bank’ batteries are popular for topping up devices on the go. Beware that some USB power banks have low capacity and charge slowly. This post explains how USB power banks work, how to test them using simple equipment and power bank sizing.
How USB power banks work
A common USB power bank contains Li-ion cells, a boost converter, a charge controller and, hopefully, a protection circuit (Li-ion cells usually do not have built-in protection).
Nominal cell voltage for Lithium nickel manganese cobalt cells is 3.7 V and maximum charging voltage is 4.2 V. Nominal USB voltage is 5 V. The boost converter steps up voltage from 3.7 V to 5 V output. The charge controller stops USB charging at 4.2 V.
The capacity rating for USB power banks usually refers to the Li-ion cells and is substantially greater than the output capacity at USB 5 V.
The output USB voltage is higher than the Li-ion cell voltage. The cell current is greater than the output USB current:
(Power out) = (Power in) × (Boost converter efficiency)
(Volts out) × (Current out) = (Volts in) × (Current in) × (Efficiency)
Substituting nominal voltages:
5 × (Current out) = 3.7 × (Current in) × (Efficiency)
(Current out) = 3.7 × (Current in) × (Efficiency) ÷ 5
(Capacity out) = 3.7 × (Capacity in) × (Efficiency) ÷ 5
where capacity is measured in mAh. Substituting the nominal 3.7 V voltage is slightly inaccurate. The Li-ion voltage actually declines during discharge to about 3 V when empty (see example below).
Substituting DC-to-DC switching converter efficiency of between 75% and 98%:
(Capacity out) = 3.7 × (Capacity in) × (0.75 to 0.98) ÷ 5
(Capacity out) = (0.56 to 0.73) × (Capacity in)
This equation is important because the mAh at USB 5 V (see block diagram above for measuring points) is substantially smaller than the mAh drawn from the Li-ion cell.
Known-capacity device testing method
Charging a known-capacity device is the cheapest method to test discharge capacity of a USB power bank. My smart phone is new, the battery is new and the battery meter seems to be reasonably accurate:
Discharge capacity = (End % − Start %) × (Device capacity) ÷ 100
Start % = Device battery meter when a full-charged power bank is plugged in to the device.
End % = Device battery meter when the power bank is empty.
Device capacity is for 3.7 V nominal Li-ion cell voltage, the same voltage as the power bank cell.
The factor 100 is used to convert percentages to fractions.
Average discharge rate = (Discharge capacity) ÷ (Discharge duration)
To discharge in one step, the device capacity should be greater than the USB power bank capacity at 5 V (see previous section). The device should be switched off, so that it drawing current for charging and little else. This method will be unreliable for devices with old batteries having reduced and unknown capacity. This method does not measure voltage.
USB meter testing methods
USB meters can be used to measure both discharging and charging. I purchased a simple USB voltage and current meter on ebay for AUD 2.
To measure capacity, I record time, voltage and current at regular intervals, enter the data in a spreadsheet and integrate current over time. Longer sampling intervals can be used for high capacity batteries. Accuracy is limited by the meter accuracy (±2 % current accuracy for my meter) and sampling frequency.
I later purchased a USB voltage, current and capacity meter for 5 AUD, which integrates current over time and reports capacity.
Example: Laser 2200 mAh Powerbank
The Laser 2200 mAh USB power bank is popular in Australia, where it retails for between AUD 8 and AUD 15.
I discharged the Laser power bank into my phone. Average output voltage of 4.96 V was very close to 5 V nominal USB voltage. Average current of 684 mA was typical for my phone. Maximum current of 870 mA was close to rated 1 A output for this power bank. Integrated discharge capacity was 1254 mAh. My phone’s battery meter indicated 1383 mAh. Another discharge test with an integrating meter gave 1230 mAh. Converting to 3.7 V nominal Li-ion cell voltage and assuming 75% to 98% boost converter efficiency (see introduction above), the estimated cell capacity was 1718 to 2240 mAh. This range includes the 2200 mAh rated capacity.
I charged the Laser power bank with a mains charger (maximum rated output current 1 A at 5 V). The average charging current of 503 mA agreed with 500 mA rated charging current. Integrated charging capacity was 2305 mAh (no voltage conversion is applied to charge capacity because current out = current in). Assuming 80% to 90% Li-ion charge/discharge efficiency, the estimated cell capacity was 1844 to 2075 mAh, which agrees with the discharge capacity.
This Laser power bank performed as rated but the low capacity charges my phone only 50%. Nearly identical USB power banks can be purchase on ebay for AUD 1, excluding the battery. High capacity 2900 to 3600 mAh Panasonic 18650 batteries cost about AUD 10. We can buy these componenets and assemble a higher capacity power bank for about the same price as the 2200 mAh Laser power bank.
Sizing USB power banks
From the introduction:
(Capacity out) = (0.56 to 0.73) × (Capacity in)
This capacity equation can be used for sizing USB power banks (i.e. capacity of the Li-ion cells). Li-ion charge/discharge efficiency is between 80 and 90%:
(Device capacity) = (0.80 to 0.90) × (Capacity out)
(Device capacity) = (0.80 to 0.90) × (0.56 to 0.73) × (Rated capacity)
(Device capacity) = (0.80 × 0.56 to 0.73 × 0.90) × (Rated capacity)
(Rated capacity) = (1.53 to 2.25) × (Device capacity)
An easy to remember rule of thumb is (USB power bank rated capacity)= 2 × (Device capacity). Now you understand why USB power banks often seem to be inadequate!